R1:643d8070f15b

Implement improved remarkup assistance panel

Summary:
- I made a "?" icon for help/reference.
- The `<>` icon was slightly too wide so I carved it down to 14x14.
- All the icons are in `/Phabriactor/remarkup_icon_sources.psd` if you want to tweak anything.
- Tooltips don't look like the mock but I'll tackle those separately.
- Removed strikethrough.
- Removed tag/image/text size for now since they don't have reasonable JS implementations yet.
- I think everything else is accurate to the mock.